Diagnosis
This is the classic 'motor hums but pump doesn't engage' problem. The most likely cause is that the pump is air-locked, which happens a lot after storage or if you started it without water first.
How to Fix It
First, turn the machine off and unplug it. Disconnect the high-pressure hose from the pump outlet. Point the hose into a bucket. Now, turn on the water supply at the tap fully and let water run through the machine and out the disconnected hose for about 30 seconds to purge all the air. Reconnect the hose, plug the machine back in, and try it. You should hear the pump 'catch' and start building pressure properly.
If That Didn't Work
If it's still just humming, the pump's ceramic pistons might be stuck from mineral deposits or sitting dry. You'll need to remove the pump head (four bolts) and check the three pistons and cylinders for scaling or obstruction. A good soak in white vinegar can dissolve light scale.
Heads Up
Never, ever start the pressure washer without water flowing through it first. Running it dry for even a few seconds can score the pistons and cylinders, which is a much more expensive fix.